The invention relates to the diagnosis of variants of the human immunodeficiency virus (HIV), in particular of the type 1 virus (HIV-1). The human immunodeficiency virus, which by now may be better defined as a group of related viruses, is the causative agent for the acquired immuno deficiency syndrome, better known as AIDS.
Apart from the variety of viruses which now have been identified as HIV species, there is also a variety within a single virus species, such as HIV-1.
HIV-1 phenotype variability plays a keyrole in the pathogenesis of AIDS. In early asymptomatic infection, virus isolates are generally slow replicating, macrophage-tropic and non syncytium inducing (NSI). In the course of infection, isolates with reduced macrophage-tropic capacity and increased replication rates become predominant (M. Tersmette et al. J. Virol. 63:2118-2125 1989, H. Schuitemaker et al. J. Virol. 65:356-363 1991, H. Schuitemaker et al. J. Virol. 66:1354-1360 1992). In 50% of infected individuals, syncytium inducing (SI) variants arise, coinciding with an accelerated depletion of CD4 positive T-lymphocytes and rapid disease progression (M. Koot et al. Ann. Int. Med. 118:681-688 1993). In addition, survival time after AIDS diagnosis is reduced for persons with SI isolates (M. Tersmette et al. Lancet i:983-985 1989).
Monitoring SI capacity of HIV-1 isolates in seropositive individuals may be used as a prognostic marker for rate of CD4 cell decline and rapid progression to AIDS. The relative risk for disease progression for persons with SI variants is higher than the relative risk of i.e. low CD4 cell numbers or p24 antigenemia and is independent of the latter two markers (M. Koot et al. Ann. Int. Med. 118:681-688 1993). Apart from the role of HIV-1 biological variability in the natural course of infection, efficacy of zidovudine treatment appeared to be dependent on the biological phenotype of HIV-1. In a study of zidovudine treated asymptomatic individuals at high risk for progression to AIDS, only transient improvement in CD4 cell numbers and response to anti-CD3 monoclonal antibodies in the zidovudine treated persons was observed. However, zidovudine treatment delayed clinical progression significantly in persons who did not develop SI variants (M. Koot et al. J. Infect. Dis. 168:733-736 1993). Therefore, it is important to determine HIV-1 phenotype of participants in anti-viral drug trials for proper randomization to improve evaluation of efficacy of anti-viral therapy.
So far, SI capacity of isolates from seropositive individuals was monitored using either PHA stimulated peripheral blood mononuclear cells (PBMC) or the MT2 T-cell line as indicator. It was shown that the ability to induce syncytia in PHA stimulated PBMC correlated with the ability to replicate in the MT2 T-cell line for all HIV-1 isolates tested (M. Koot et al. AIDS 6:49-54 1992). Both methods require specialized laboratory equipment and well trained laboratory personnel since HIV-1 is propagated to high titers. Moreover, the methods require use of purified lymphocytes and are less applicable to whole blood cell samples or serum and plasma samples. In individuals with extremely low CD4 cell numbers (i.e. after AIDS diagnosis) the methods have limited value.
In the search for molecular determinants of HIV-1 SI capacity it was found that the third variable domain (V3) of the envelope molecule gp120 plays an important role in determining SI capacity. Exchange of V3 domains between SI and NSI molecular clones of HIV-1 demonstrated that V3 can confer SI capacity (M. Groenink et al. Science 260:1513-1516 1993). Sequence analysis of V3 domains from a set of naturally occurring HIV-1 isolates revealed a nearly 100% correlation between HIV-1 phenotype and aminoacid composition of V3 (R. Fouchier et al. J. Virol. 66:3183-3187 1992). In SI HIV-1 isolates the V3 domain harbours positively charged aminoacid residues (Arginine or Lysine) at positions 306 or 320. In NSI HIV-1 isolates both residues are either uncharged or negatively charged. Site directed mutagenesis at these positions in V3 revealed that presence of positively charged residues at either position indeed confers SI capacity (J. De Jong et al. J. Virol. 66:757-765 1992).
The present invention solves the problem of how to make use of these data to discriminate between HIV-1 isolates which do and HIV-1 isolates which do not induce syncytia. The invention provides a method for discriminating between syncytia inducing and non-syncytium inducing variants of the human immunodeficiency virus, wherein the ability to induce syncytium is determined by identifying characterizing nucleotides therefore in either the coding or the non coding strand of the virus. Though the prevalence of the amino acids in HIV-SI variants in comparison with HIV-NSI variants was already known, it was not envisaged how this could be used to discriminate between the two. By Analyzing the difference at the nucleic acid level this discrimination has now become possible in a rapid, easy and safe manner.
In its most general form, the invention described herein presents oligonucleotides with sequences specific for SI HIV-1 isolates that may be used for determination of HIV-1 phenotype in biological materials obtained from seropositive individuals. These oligonucleotides may be variable in length and nucleotide composition, dependent on the technique used for detection, but meet the criteria to discriminate SI from NSI strains based on DNA-sequences responsible for phenotype-specific aminoacid variation at positions 306 and 320 in V3.
In order to determine SI-specific DNA sequence variation first V3 amino acid sequences from 251 isolates obtained from 90 individuals were analyzed for correlation between charge of aminoacid residues at positions 306 and 320 and SI capacity. This correlation was found to be nearly 100% as described earlier (R. Fouchier et al. J. Virol. 66:3183-3187 1992) and would therefore allow prediction of viral phenotype. Next, we analyzed the frequency of amino acid substitutions at positions 306 and 320 in V3 from SI and NSI isolates and codon usage at these positions, to investigate the phenotype-restricted DNA sequence variation that encodes phenotype-restricted aminoacid variation. In NSI isolates, a Serine or Glycine residue is present at position 306 in nearly 100% of the virus isolates. In SI isolates, this residue is substituted by an Arginine residue in 48% of the sequences. At position 320 3% of the NSI isolates has a Lysine or Arginine residue whereas in the group of SI viruses 43% has such positively charged residues at this position. Given the relatively restricted aminoacid sequence variation at positions 306 and 320, we emphasized that limited DNA sequence variation could account for differences in SI capacity. Analysis of the DNA sequences that encode the amino acid residues at positions 306 and 320 is shown. Although Arginine residues may be coded for by 6 different codons, presence of Arginine residues at position 306 in V3 results from either an AGG or AGA codon in all SI isolates. In NSI isolates, the last nucleotide of the codon at position 306 is either a T or a C (i.e. AGT and AGC encoding Serine and GGT encoding Glycine for 190 out of 191 NSI isolates). Presence of the Arginine and Lysine residues at position 320 results from codons AAA (Lysine) and AGA (Arginine) in all SI isolates. In 181 out of 191 NSI sequences, either G or C is the first nucleotide in codon 320 (i.e. GAA and GAG encoding Glutamic acid, GAT and GAC encoding aspartic acid, and CAA encoding Glutamine). By using an assay according to the present invention that would allow for discrimination of A and G from C and T residues as the last nucleotide in codon 306 and A from G, C and T residues as the first nucleotide in codon 320, analysis of virus isolates from the 90 individuals in this dataset would have resulted in a proper determination of viral phenotype in 94% of the individuals.
There are a number of ways in which the discrimination between A or G and C or T at codon 306 as well as the discrimination between A and C or G or T at codon 320 can be carried out, a suitable one being described in the experimental part below. Although the invention is explained in detail for the discrimination between SI and NSI variants of HIV-1, it will be clear that a similar discriminative may be found in other HIV species, such as HIV-2. The contribution of this invention to the art, in one part, is the realization that though at the amino acid level a difference may be observable in SI variants as compared to NSI variants, but that this can be exploited to discriminate between the two when applied to the nucleic acid level. This should be broader applicable than for HIV-1 only. On the other hand, it is also a part of the invention how to discriminate between the SI and NSI variants, for which a very elegant method is disclosed in the experimental part.
Usually the methods for discrimination between HIV-SI and HIV-NSI will employ hybridization of an oligonucleotide to either the coding or the non coding strand of the V3 loop of the GP120 protein. These methods will in one way or another make use of the fact that mismatches will occur when hybridizing an oligonucleotide complementary to an SI variant to an NSI variant DNA molecule or vice versa. These hybridization assays are well known in the art and suitable ones such as northern blots, southern blots and in situ hybridization can be selected. They can include, but are not limited to hybridization techniques in which the hybrids are somehow detected, either through attaching a label to the oligonucleotide, or through specific binding reactions between hybrids and reactants provided with a label. In this respect it should be noted that a label is intended to mean any detectable moiety, be it through further reactions, or directly observable in any suitable way.
Suitable hybridization techniques also include the well known amplification techniques, such as NASBA, PCR and the like, in which sets of oligonucleotides (primers) are hybridized to the desired nucleic acid a part of which is to be identified and which serve as a template for an enzyme such as a DNA polymerase which reproduces multiple copies of the part of the nucleic acid of interest. The selectivity of such methods for a large part depends on the selection of suitable primers. The present invention provides novel primers which have been very elegantly chosen to allow for the discrimination between SI and NSI variants. It is often sufficient to use just one of the novel specific primers according to the invention, as disclosed in the experimental part, with one of the primers complementary to a more conserved region, but many suitable combinations can be designed by the man skilled in the art without departing from the spirit of the present invention. For instance a sequence at least 70% homologous with, a primer according to the invention or a sequence which hybridizes to such a sequence or its complementary counterpart under stringent conditions can be used. The invention will now, for illustration purposes only, be explained in more detail in the following experimental part.